U.S. patent number 6,118,218 [Application Number 09/241,882] was granted by the patent office on 2000-09-12 for steady-state glow-discharge plasma at atmospheric pressure.
This patent grant is currently assigned to Sigma Technologies International, Inc.. Invention is credited to Wolfgang Decker, Shahid A. Pirzada, Angelo Yializis.
United States Patent |
6,118,218 |
Yializis , et al. |
September 12, 2000 |
Steady-state glow-discharge plasma at atmospheric pressure
Abstract
A plasma treater incorporates a porous metallic layer in one of
the electrodes. The porous layer is selected with pores of average
size within one order of magnitude of the mean free path of the
plasma gas at atmospheric pressure. The plasma gas is injected into
the electrode at substantially atmospheric pressure and allowed to
diffuse through the porous layer, thereby forming a uniform
glow-discharge plasma. The film material to be treated is exposed
to the plasma created between this electrode and a second electrode
covered by a dielectric layer. Because of the micron size of the
pores of the porous metal, each pore also produces a hollow cathode
effect that facilitates the ionization of the plasma gas. As a
result, a steady-state glow-discharge plasma is produced at
atmospheric pressure and at power frequencies as low as 60 Hz.
Inventors: |
Yializis; Angelo (Tucson,
AZ), Pirzada; Shahid A. (Tucson, AZ), Decker;
Wolfgang (Tucson, AZ) |
Assignee: |
Sigma Technologies International,
Inc. (Tucson, AZ)
|
Family
ID: |
22912549 |
Appl.
No.: |
09/241,882 |
Filed: |
February 1, 1999 |
Current U.S.
Class: |
315/111.21;
118/723E; 219/121.36 |
Current CPC
Class: |
H01J
37/32009 (20130101); H01J 37/32532 (20130101); H01J
37/32449 (20130101); H01J 37/3244 (20130101) |
Current International
Class: |
H01J
37/32 (20060101); H05H 001/24 () |
Field of
Search: |
;315/111.21,111.31
;313/231.13,231.41 ;219/121.36 ;118/723E,723ER |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bettendorf; Justin P.
Attorney, Agent or Firm: Durando; Antonio R.
Claims
What is claimed is:
1. Apparatus for producing a glow-discharge plasma at substantially
atmospheric pressure, comprising the following combination of
components:
a pair of opposing electrodes operating at substantially
atmospheric pressure, at least one of the electrodes comprising a
metallic porous layer;
means for applying a voltage across the electrodes; and
means for diffusing a plasma gas through said metallic porous
layer.
2. The apparatus of claim 1, further comprising a dielectric layer
placed between said opposing electrodes.
3. The apparatus of claim 2, wherein said electrode comprising a
metallic porous layer includes an enclosed tubular structure
containing a metallic porous portion, and said means for diffusing
a plasma gas through the metallic porous layer includes a conduit
into the enclosed tubular structure.
4. The apparatus of claim 3, further comprising a baffle within
said enclosed tubular structure, said baffle containing a plurality
of perforations adapted to produce a substantially uniform flow of
the plasma gas to the metallic porous portion.
5. The apparatus of claim 4, wherein said plasma gas has a mean
free path at atmospheric pressure and the metallic porous portion
has pores with an effective diameter substantially within one order
of magnitude greater than said mean free path.
6. The apparatus of claim 1, further comprising means for passing a
web through said glow-discharge plasma in order to enhance a
surface characteristic of the web.
7. The apparatus of claim 1, wherein said plasma gas has a mean
free path at atmospheric pressure and the metallic porous layer has
pores with an effective diameter substantially within one order of
magnitude greater than said mean free path.
8. The apparatus of claim 7, wherein said plasma gas comprises
helium and said pores have an effective diameter smaller than 20
microns.
9. The apparatus of claim 1, wherein said electrode comprising a
metallic porous layer includes an enclosed tubular structure
containing a metallic porous portion, and said means for diffusing
a plasma gas through the metallic porous layer includes a conduit
into the enclosed tubular structure.
10. The apparatus of claim 9, further comprising a baffle within
said enclosed tubular structure, said baffle containing a plurality
of perforations adapted to produce a substantially uniform flow of
the plasma gas to the metallic porous portion.
11. The apparatus of claim 10, wherein said plasma gas has a mean
free path at atmospheric pressure and the metallic porous portion
has pores with an effective diameter substantially within one order
of magnitude greater than said mean free path.
12. Apparatus for producing a glow-discharge plasma at
substantially atmospheric pressure, comprising the following
combination of components:
a pair of opposing electrodes;
a metallic wire-cloth layer attached to at least one of the
electrodes;
means for applying a voltage across the electrodes; and
means for diffusing a plasma gas through said electrode
wherein said metallic wire-cloth layer is at least as fine as 200
mesh.
13. The apparatus of claim 12, wherein said metallic wire-cloth
layer is wrapped over said at least one of the electrodes.
14. The apparatus of claim 12, further comprising means for
diffusing the plasma gas through said metallic wire-cloth
layer.
15. The apparatus of claim 14, further comprising means for passing
a web through said glow-discharge plasma in order to enhance a
surface characteristic of the web.
16. A method for producing a glow-discharge plasma at substantially
atmospheric pressure, comprising the following steps:
providing a pair of opposing electrodes operating at substantially
atmospheric pressure, at least one of the electrodes comprising a
metallic porous layer;
applying a voltage across the electrodes; and
diffusing a plasma gas through said metallic porous layer.
17. The method of claim 16, further comprising the step of placing
a dielectric layer between said opposing electrodes.
18. The method of claim 16, wherein said electrode comprising a
metallic porous layer includes an enclosed tubular structure
containing a metallic porous portion, and said step of diffusing a
plasma gas through the metallic porous layer is carried out through
a conduit feeding the enclosed tubular structure.
19. The method of claim 18, further comprising a baffle within said
enclosed tubular structure, said baffle containing a plurality of
perforations adapted to produce a substantially uniform flow of the
plasma gas to the metallic porous portion.
20. The method of claim 16, wherein said plasma gas has a mean free
path at atmospheric pressure and the metallic porous layer has
pores with an effective diameter substantially within one order of
magnitude greater than said mean free path.
21. The method of claim 16, wherein said plasma gas comprises
helium and said pores have an effective diameter smaller than 20
microns.
22. The method of claim 16, further comprising the step of passing
a web through said glow-discharge plasma in order to enhance a
surface characteristic of the web.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to methods and apparatus for
producing plasma; in particular, the invention relates to
establishing a steady-state glow-discharge plasma at atmospheric
pressure and low temperatures.
2. Description of the Related Art
Plasma is an ionized form of gas that can be obtained by ionizing a
gas or liquid medium using an AC or DC power source. A plasma,
commonly referred to as the fourth state of matter, is an ensemble
of randomly moving charged particles with sufficient density to
remain, on average, electrically neutral. Plasmas are used in very
diverse processing applications, ranging from the manufacture of
integrated circuits for the microelectronics industry, to the
treatment of fabric and the destruction of toxic wastes.
In particular, plasmas are widely used for the treatment of organic
and inorganic surfaces to promote adhesion between various
materials. For example, polymers that have chemically inert
surfaces with low surface energies do not allow good bonding with
coatings and adhesives. Thus, these surfaces need to be treated in
some way, such as by chemical treatment, corona treatment, flame
treatment, and vacuum plasma treatment, to make them receptive to
bonding with other substrates, coatings, adhesives and printing
inks. Corona discharge, physical sputtering, plasma etching,
reactive ion etching, sputter deposition, plasma-enhanced chemical
vapor deposition, ashing, ion plating, reactive sputter deposition,
and a range of ion beam-based techniques, all rely on the formation
and properties of plasmas.
Corona discharges are widely used in particular for treating
plastic films, foils, papers, etc. to promote adhesion with other
materials by increasing the surface energy of the film. A corona
discharge is established between two electrodes by applying a high
voltage to one of the electrodes while the other is connected to
ground at typical frequencies in the order of 10-50 kHz. These
conditions produce locally concentrated discharges known in the art
as streamers, which lead to some non-uniformity in the treatment of
film surfaces and can also damage the film by producing low
molecular weight species that adversely affect adhesion to the
surface. Furthermore, the streamers of corona treatment can produce
backside effects on the film being treated, which is undesirable in
many applications. Nevertheless, corona treatment is extensively
used in the industry for improving the surface energy of
materials.
Glow-discharge plasma treatment is also an effective method of
treating surfaces to increase their wettability and adhesion to
various materials. Glow discharge provides a more uniform and
homogenous plasma that produces a more consistent surface treatment
than corona treatment, thereby avoiding unintentional back
treatment of the film. Glow-discharge plasma is characterized by
high-energy electrons that collide with, dissociate and ionize
low-temperature neutrals, creating highly reactive free radicals
and ions. These reactive species enable many chemical processes to
occur with otherwise unreactive low-temperature feed stock and
substrates. Based on these properties, low-density glow-discharge
plasmas are usually utilized for low material-throughput processes
involving surface modification. These plasmas are typically formed
by partially ionizing a gas at a pressure well below atmosphere.
For the most part, these plasmas are weakly ionized, with an
ionization fraction of 10.sup.-5 to 10.sup.-1, established with AC
or DC power in systems with varied geometries. These systems always
require vacuum chambers and pumps to maintain a low pressure, which
increases operating costs and maintenance.
There has been an extensive effort to develop plasma systems
capable of operating at atmospheric pressure for surface treatment
of polymer films, foils, and paper, in order to avoid capital and
maintenance expenditures for vacuum chambers and pumps. It is known
that atmospheric plasma can be generated at relatively low
temperatures with a proper power source, the insertion of a
dielectric layer between the electrodes, and the use of an
appropriate gas mixture as plasma medium. For surface treatment of
polymer films, fabrics, paper, etc., atmospheric plasma can be
established between two electrodes using an inert gas such as
helium under particular operating conditions. Usually one electrode
is attached to a high voltage power supply, and a rotating drum is
grounded and acts as the other electrode. One electrode is coated
with a ceramic layer and the plasma gas is injected between
electrodes.
For example, U.S. Pat. No. 5,456,972 describes a glow-discharge
plasma system operating at atmospheric pressure. The apparatus
consists of a pair of spaced plate electrodes energized over a
range of about 1 to 5 KV rms at a radio frequency of about 1 to 100
KHz. A plasma gas is injected between the plates and the film to be
treated is passed through them and exposed to the resulting plasma
discharge for a predetermined period of time.
U.S. Pat. No., 5,789,145 discloses an atmospheric-pressure
glow-discharge system based on conventional corona discharge
apparatus. The improvement consists of pumping a gas comprising
mostly helium through a gas delivery system containing slits that
distribute the flow uniformly to the electrode arrangement. Various
other patents describe particular gas compositions that enable the
production of atmospheric or near-atmospheric pressure
glow-discharge plasma for particular applications. See, for
example, U.S. Pat. Nos. 5,387,842, 5,403,453, 5,414,324, 5,558,843,
5,669,583, 5,767,469, and 5,789,145.
In an attempt to improve the uniformity of the plasma created at
atmospheric pressure and avoid the formation of streamers typical
of corona treatment, perforated electrodes and screens have also
been used. For example, U.S. Pat. No. 5,714,308 teaches a method
for establishing a uniform atmospheric plasma between two
electrodes using an inert gas or a mixture of gases as plasma
medium. One of the electrodes is connected to an AC power supply.
The other electrode is covered with an insulating material. As is,
such an electrode arrangement will produce a corona discharge if
powered by a voltage at 40-500 kHz. The invention consists of
providing uniformly spaced holes in one electrode with a diameter
of the order of 1 mm and pumping a gas through the holes, which has
been found to produce atmospheric glow-discharge plasma at these
frequencies using mixtures of helium and other gases, such as
N.sub.2 and O.sub.2, so long as such other gases do not exceed
8.0%.
Thus, it has been shown that a uniformly perforated structure
placed between electrodes improves gas diffusion and,
correspondingly, makes it possible to obtain some level of
glow-discharge plasma at higher pressures than previously possible.
Other than under very specific operating conditions and limited gas
compositions, though, these system cannot produce a uniform glow
discharge at atmospheric pressure.
Therefore, there is still a need for a plasma treatment system
capable of producing a steady glow discharge at atmospheric
pressure with different gas mixtures. In addition, it would be
desirable to be able to operate such a system at frequencies as low
as 60 Hz.
BRIEF SUMMARY OF THE INVENTION
One primary objective of this invention is a method and apparatus
for producing a glow-discharge plasma at atmospheric pressure.
Another important goal of the invention is a method and apparatus
that provides a glow discharge under steady-state conditions.
Another objective of the invention is a procedure that produces a
relatively low-temperature plasma.
Another objective is a process that can be carried out using a
power system operating at relatively low frequencies.
Still another objective is a method and apparatus that are suitable
for incorporation with existing plasma equipment.
A final objective is a procedure that can be implemented easily
and
economically according to the above stated criteria.
Therefore, according to these and other objectives, the preferred
embodiment of the invention consists of incorporating a porous
metallic layer in one of the electrodes of a plasma treatment
system. A plasma gas is injected into the electrode at
substantially atmospheric pressure and allowed to diffuse through
the porous layer, thereby forming a uniform glow-discharge plasma.
As in prior-art devices, the film material to be treated is exposed
to the plasma created between this electrode and a second electrode
covered by a dielectric layer. Because of the micron size of the
pores of the porous metal, each pore also produces a hollow cathode
effect that facilitates the ionization of the plasma gas. As a
result, a steady-state glow-discharge plasma is produced at
atmospheric pressure and at power frequencies as low as 60 Hz.
Various other purposes and advantages of the invention will become
clear from its description in the specification that follows and
from the novel features particularly pointed out in the appended
claims. Therefore, to the accomplishment of the objectives
described above, this invention consists of the features
hereinafter illustrated in the drawings, fully described in the
detailed description of the preferred embodiment and particularly
pointed out in the claims. However, such drawings and description
disclose only some of the various ways in which the invention may
be practiced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of an atmospheric plasma
treater 10 according to the invention.
FIG. 2 is a partially cut-out, side sectional view of an electrode
containing a porous-metal component according to the preferred
embodiment of the invention, shown as seen from line 2--2 in FIG.
3.
FIG. 3 is a front sectional view of the electrode of FIG. 2 as seen
from line 3--3 in that figure.
FIG. 4 illustrates in front view a plasma treater with three
elements of a porous electrode according to the invention.
FIG. 5 is a schematic representation of the embodiment of the
invention using an electrode containing a porous-metal strip.
FIG. 6 is a schematic representation of the embodiment of the
invention using an electrode containing a porous wire-cloth
strip.
FIG. 7 is a schematic representation of the embodiment of the
invention using an electrode a porous wire-cloth strip wrapped over
a conventional electrode.
FIG. 8 is a perspective view of the plasma treater of the invention
placed on a conventional web roller.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The heart of this invention lies in the incorporation a
porous-metal layer in one of the electrodes of an atmospheric
plasma treatment unit and the forced diffusion of the plasma medium
through such porous structure. During experimental work performed
to improve prior-art results obtained with perforated electrodes
and/or wire mesh for diffusing the flow of plasma gas, the
inventors realized that, in order for the electrode holes to
operate effectively for producing glow discharge, their size must
approach the mean free path of the plasma gas at the system's
operating pressure. As commonly understood in the art, for the
purposes of this disclosure the terms size and effective size of
the pores refer to a hypothetical diameter equal to the geometric
mean of the largest and smallest dimension of the pore.
At atmospheric pressure, the mean free path of all gases typically
used to produce plasma is in the submicron to micron range. Initial
attempts with a metallic mesh screen (mesh size 600-200) produced
significant improvements over the prior art, which are considered
encompassed by the present invention. However, even a 600-mesh
screen produces holes that are more than 20 microns in diameter,
which is considerably larger than the mean free path of plasma
gases at atmospheric pressure. Helium, for example, has a mean free
path of about 0.13 micron at one atmosphere. A search for ways to
produce micron- and submicron-size holes led to the idea of
constructing an electrode out of porous metal, such that the size
of the pores could be controlled by the proper choice of the
metal-grain size used to produce the electrode.
As those skilled in the art would readily understand, a porous
metal is a material produced by powder metallurgy. Metal powder
particles of desired size are blended with a binder, if necessary,
then pressed and sintered to produce a solid porous structure. The
pore size is controlled by the size of the powder grains used as
raw material. The term "porous metal," as used in this disclosure,
is intended to refer to such materials.
By judiciously selecting the grain size and the type of metal used
for producing the porous metallic layer, the electrode design of
the invention can provide a structure with densely and uniformly
distributed holes as small as 0.1 micron in diameter. The high
density of holes in the electrode creates a uniform high-density
plasma that prevents corona streamer formation and allows the use
of much broader ranges of voltage frequencies and gas mixtures than
previously possible. The submicron/micron size of the holes
approximates the mean free path of the plasma gas and it is
believed to allow each pore hole to act as a hollow cathode during
the negative part of the voltage cycle. This generates intense
plasma that in turn enhances the level of treatment.
We found that the ideal pore size for a particular application is
approximately within one order of magnitude greater than the mean
free path of the plasma medium at the chosen operating conditions.
Thus, working with helium as the main plasma-gas component, we
found that a steady atmospheric glow discharge could be sustained
easily even at very low frequencies using porous metals with pores
up to 10 microns in size. As the size of the pores increased over
approximately 10 microns in diameter, a uniform plasma could only
be produced with a high content of helium and at higher
frequencies.
Referring to the drawings, wherein like parts are designated
throughout with like numerals and symbols, FIG. 1 shows a general
layout of an atmospheric plasma treater 10 according to the
invention. The plasma treater 10 is shown mounted on the roller 12
of a conventional web-treatment system. A film 14 of material to be
treated is passed through the system between the plasma treater and
the roller at speeds typically ranging from 1 to 200 ft/min. The
roller 12 is grounded and coated with a dielectric material 16 such
as polyethylene teraphthalate (PET). Other materials with different
dielectric constants ranging from 3 to 1000, such as alumina,
silica, barium titanate, and strontium titanate, could be used as
equivalent dielectric material. The plasma treater 10 contains at
least one electrode according to the invention (shown in FIG. 2)
which is connected, through a cable 18, to an AC power supply 20
operating at any frequency between 60 Hz and the maximum frequency
available from the power supply. The treater is held in place
conventionally by a holding bracket 22 to maintain a distance of
1-2 mm between the roller 12 and the treater 10. This distance,
which varies with operating conditions, plasma medium composition,
and electrode configuration, is important in establishing a steady
plasma flow; therefore, it is very desirable to maintain a gap
determined to be optimal. Plasma gas, such as helium, argon, and
mixtures of an inert gas with nitrogen, oxygen, air, carbon
dioxide, methane, acetylene, propane, ammonia, or mixtures thereof,
can be used with this treater to sustain a uniform and steady
plasma. The gas is supplied to the treater 10 through a manifold 24
that feeds the porous electrode of the invention.
FIGS. 2 and 3 illustrate in side and front elevational view,
respectively, a plasma treater electrode 30 according to a
preferred embodiment of the invention. The electrode consists of a
closed-ended tubular structure preferably constructed by enclosing
a hollow metallic housing 32 with a porous metal layer 34 having
pores sized to approximate the mean free path of the plasma gas
intended to be used in the treater. The gas is fed to the upper
portion 36 of the hollow electrode 30 at substantially atmospheric
pressure through an inlet pipe 38 connected to the exterior
manifold 24. Similarly, the electrode is energized by an electrical
wire 40 connected to the power system through the exterior cable
18. The electrode 30 preferably includes a distribution baffle 42
containing multiple, uniformly spaced apertures 44 designed to
distribute the gas uniformly throughout the length of the bottom
portion 46 of the hollow electrode 30. While the presence of the
baffle 42 is not critical to the invention, as a result of this
configuration the gas pressure against the bottom porous layer 34
can be maintained uniform and consistent through feed fluctuations,
which contributes to the production of a uniform glow-discharge
plasma.
FIG. 4 illustrates in front view a plasma treater 10 (without the
front cover 48 seen in FIG. 1) with three elements of a porous
electrode 30 according to the invention. These are preferably made
of stainless steel, such as SS316, but the porous layer 34 of the
electrode can alternatively be made of other material that may be
better suited to produce the desired porous size for a particular
application. Each electrode element is held in place by a ceramic
insulator 50 and a metallic support 52 attached to the cover 54 of
the plasma treater. Each electrode element is typically provided
with a separate height adjustment mechanism (not shown in the
figures) to permit its alignment with the roller 12. The cover 54
of the plasma treater preferably extends on both sides over the
roller 12 to minimize ambient air inflow into the plasma
treater.
The baffle 42 separating the upper and lower portions 36,46 of each
electrode 30 preferably consists of a stainless steel plate with
1-mm holes spaced every 10 cm along the main axis of the plate. The
stainless steel porous metal strip 34 is attached to the bottom of
the electrode to form an enclosed structure. The thickness of the
porous metal layer 34 can vary to produce the desired plasma flow,
depending on the pressure of the gas feed and the size and density
of the pores, but thicknesses from about 1.5 mm to 6.5 mm have been
shown to yield very good results. Porous metals such as these are
used in other applications such as for filter elements of gas lines
and ball bearings, and are commercially available from companies
such as the Mott Corporation of Farmington, Conn. These commercial
porous metals have pores ranging from 0.1 to 20 microns in
diameter.
In another embodiment of the invention, which is not preferred but
was tested with significant success before we realized the
advantages of using porous metals, a metal cloth with very tight
mesh was used to enclose the hollow metallic housing 32 of the
electrode 30. The configuration of the plasma treater 10 was
retained in all other respects. As shown by the examples reported
below, this wire-cloth embodiment provided noticeable improvements
over the prior art but, inasmuch as it still produced some
streamers, it did not yield a steady glow discharge at atmospheric
pressure. Earlier work with the same very fine metal cloth (up to
600 mesh was tried) wrapped over a conventional electrode also
produced an acceptable glow discharge with some streamers, but
better results were obtained by passing the gas through the wire
mesh. All embodiments of the invention are illustrated
schematically in FIGS. 5, 6 and 7 for a perfused porous-metal strip
34, a perfused wire-cloth strip 54, and a wrapped wire-cloth strip
56, respectively. The actual plasma treater 10 is shown placed on a
web roller 12 in the overview of FIG. 8.
The plasma treater of the invention was used for etching polymeric
substrates and for treating biologically contaminated materials.
Using different porous metals and wire cloths, the openings in the
electrode were varied from about 0.1 microns to about 50 microns in
effective diameter. We confirmed the hypothesis that having pores
of appropriate size is the key to sustaining a uniform and steady
plasma at atmospheric conditions. Based on our experiments, the
optimal pore size appears to be in the range between the mean free
path of the plasma and 10 to 20 times that size, best results being
produced up to about 10 times the mean fee path. Such porous
material of the electrode allows extremely uniform gas flow
distribution along the whole area of the electrode and provides
numerous extremely small cavities that are believed to also act as
hollow cathodes, thereby enhancing plasma intensity.
Organic films such as polypropylene, polyethylene, and polyethylene
teraphthalate substrates, commonly used in the food-packaging
industry, were treated in the plasma treater of the invention at
atmospheric conditions. Various AC-voltage frequencies were used in
the 60 Hz to 20 kHz range without noticeable difference in the
results. Surface energies of these films were substantially
enhanced by the plasma treatment. Table 1 reports the processing
conditions and the range of plasma compositions for some of the
tests conducted with the atmospheric plasma treater of the
invention to obtain a steady and uniform plasma. The remarks
adjacent to the data exemplify the results.
TABLE 1
__________________________________________________________________________
Power Pore Input, size, Gas, % W Metal .mu. He Ar O.sub.2 CO.sub.2
N.sub.2 Air H.sub.2 CH.sub.4 NH.sub.3 Remarks
__________________________________________________________________________
200 SS316 0.2 100 0 0 0 0 0 0 0 0 uniform plasma 0-45 0-55 0 0 0 0
0 0 0 0-70 0 0-30 0 0 0 0 0 0 0 0-50 0 0 0-50 0 0 0 0 0 0-80 0 0 0
0-20 0 0 0 0 0-75 0 0 0 0 0-25 0 0 0 0-90 0 0 0 0 0 0-10 0 0 0-95 0
0 0 0 0 0 0-5 0-5 0-95 0 0 0 0 0 0 0 200 SS316 0.5 100 0 0 0 0 0 0
0 0 uniform plasma 0-70 0 0-30 0 0 0 0 0 0
200 SS316 2.0 100 0 0 0 0 0 0 0 0 uniform plasma 0-50 0 0-50 0 0 0
0 0 0 0-50 0 0 0-50 0 0 0 0 0 0-80 0 0 0 0-20 0 0 0 0 0-75 0 0 0 0
0-25 0 0 0 200 SS316 5.0 100 0 0 0 0 0 0 0 0 uniform plasma 200
SS316 10.0 100 0 0 0 0 0 0 0 0 uniform plasma 0-75 0 0 0-25 200
SS316 20.0 100 0 0 0 0 0 0 0 0 plasma and some streamers 0-75 0
0-25 0 0 0 0 0 0 200 Bronze 5.0 100 0 0 0 0 0 0 0 0 uniform plasma
200 Bronze 10.0 100 0 0 0 0 0 0 0 0 uniform plasma 200 Bronze 20.0
100 0 0 0 0 0 0 0 0 plasma and some streamers 200 SS316 37* 100 0 0
0 0 0 0 0 0 plasma and some streamers 200 SS316 44* 100 0 0 0 0 0 0
0 0 plasma and some streamers 200 SS316 22* 100 0 0 0 0 0 0 0 0
uniform plasma
__________________________________________________________________________
*mesh size, tests with screen
It is clear from these data that the use of a porous electrode
constructed at least in part with a porous metal produces a uniform
plasma at atmospheric conditions for pore sizes up to about 20
microns. Larger sizes (22, 37 and 44 microns, obtained with wire
cloth) produced a plasma with some streamers. The porous metal
electrode also produced a uniform plasma with a variety of gas
mixtures containing as low as 45 percent helium. As the data below
indicate, surface energies of the treated films were substantially
enhanced after the plasma treatment.
EXAMPLE 1
Film treated: Polypropylene (PP) film
Base surface energy: 30 dynes/cm
Web speed: 18 ft/min
Drum dielectric film: PET; Four inch plasma treater
Tables 2, 3 and 4 below show the results of plasma treatment of
polypropylene film using helium and mixtures of helium and carbon
dioxide, helium and oxygen, and helium and nitrogen in the amounts
reported.
TABLE 2 ______________________________________ Gases Power Surface
Energy, dynes/cm He CO.sub.2 Input, 20 96 168 Sample cc/min cc/min
W 0 hrs hrs hrs ______________________________________ 1 300 0 200
46 44 44 42 2 300 15 200 58 54 46 46 3 300 30 200 58 52 44 42 4 300
60 200 58 52 48 46 5 300 75 200 58 52 46 46
______________________________________
TABLE 3 ______________________________________ Gases Power Surface
Energy, dynes/cm He O.sub.2 Input, 20 75 168 Sample cc/min cc/min W
0 hrs hrs hrs ______________________________________ 1 300 7.5 200
54 52 50 48 2 300 15 200 54 50 50 48 3 300 30 200 56 52 46 44 4 300
60 200 54 52 48 42 5 300 90 200 56 52 48 46
______________________________________
TABLE 4 ______________________________________ Gases Power Surface
Energy, dynes/cm He N.sub.2 Input, 20 75 168 Sample cc/min cc/min W
0 hrs hrs hrs ______________________________________ 1 300 15 200
54 52 52 48 2 300 30 200 54 50 48 42 3 300 60 200 56 50 50 44
______________________________________
EXAMPLE 2
Film treated: Polyethylene teraphthalate (PET) film
Base surface energy: 40 dynes/cm
Drum Speed: 18 ft/min
Drum dielectric film: PET; Four inch plasma treater
Tables 5, 6 and 7 below show the results of plasma treatment of
polyethylene teraphthalate film using helium and mixtures of helium
and carbon dioxide, helium and oxygen, and helium and nitrogen in
the reported amounts.
TABLE 5 ______________________________________ Gases Power Surface
Energy, dynes/cm He CO.sub.2 Input 20 75 168 Sample cc/min cc/min W
0 hrs hrs hrs ______________________________________ 1 300 0 200 56
n/a 52 52 2 300 15 200 54 54 50 50 3 300 30 200 58 54 50 50 4 300
60 200 56 54 54 54 5 300 75 200 56 54 52 48
______________________________________
TABLE 6 ______________________________________ Gases Power Surface
Energy, dynes/cm He O.sub.2 Input, 20 75 168 Sample cc/min cc/min W
0 hrs hrs hrs ______________________________________ 1 300 15 200
52 50 50 48 2 300 30 200 52 50 48 46 3 300 60 200 52 50 50 50 4 300
90 200 54 54 52 48 ______________________________________
TABLE 7 ______________________________________ Gases Power Surface
Energy, dynes/cm He N.sub.2 Input, 48 168 Sample cc/min cc/min W 0
hrs hrs ______________________________________ 1 300 15 200 56 56
52 2 300 30 200 56 56 52 3 300 60 200 58 56 52 4 300 90 200 58 52
52 ______________________________________
EXAMPLE 3
Film Treated: Polyethylene (PE) film
Base surface energy: 30 dynes/cm
Drum Speed: 18 ft/min
Drum dielectric film: PET; Four inch plasma treater
Tables 8, 9 and 10 show the results of plasma treatment of
polyethylene film using helium and mixtures of helium and carbon
dioxide, helium and oxygen, and helium and nitrogen in the reported
amounts.
TABLE 8 ______________________________________ Gases Power Surface
Energy, dynes/cm He CO.sub.2 Input 20 168 Sample cc/min cc/min W 0
hrs hrs ______________________________________ 1 300 0 200 54 48 46
2 300 15 200 50 50 44 3 300 30 200 50 46 40 4 300 60 200 50 46 44
______________________________________
TABLE 9 ______________________________________ Gases Power Surface
Energy, dynes/cm He O.sub.2 Input 20 168 Sample cc/min cc/min W 0
hrs hrs ______________________________________ 1 300 15 200 56 48
46 2 300 30 200 54 46 44 3 300 60 200 54 44 40 4 300 90% 200 52 40
40 ______________________________________
TABLE 10 ______________________________________ Gases Power Surface
Energy, dynes/cm He N.sub.2 Input 20 168 Sample cc/min cc/min W 0
hrs hrs ______________________________________ 1 300 15 200 52 46
40 2 300 30 200 52 42 38 3 300 60 200 52 40 38
______________________________________
These results show that the apparatus of the invention can be used
for treating and modifying the surface properties of organic and
inorganic materials. This plasma-treatment system does not require
any vacuum equipment, it produces high density plasma, and the
treatment of various surfaces can be performed at low temperatures
and at atmospheric pressure. In addition, we found that a steady
glow discharge can be produced at substantially lower frequencies
than previously possible. Many tests were run routinely with
success at 1 kHz, and good glow discharge was produced at
frequencies as low as 60 Hz. In addition, the porous electrode of
the invention made it possible to obtain steady atmospheric glow
discharge with gas mixtures containing as little as 45 percent
helium, which is not possible with any prior-art device.
Thus, those skilled in the art will readily appreciate the wide
range of potential applications for this invention in areas such as
surface treatment/functionalization of polymer films for the
food-packaging industry; plasma-enhanced chemical vapor deposition
for barrier films in the packaging industry; plasma etching for the
microelectronics industry; plasma grafting and plasma
polymerization; the treatment of fabrics, wool, metal and paper;
and the sterilization of biologically contaminated materials. In
particular, surface functionalization of polymer films by plasma is
the most effective way for a uniform and controlled treatment.
Using this invention, the surface energy of the films can be
controlled by plasma treatment to enhance the wettability,
printability and adhesion of coatings.
Various changes in the details, steps and components that have been
described may be made by those skilled in the art within the
principles and scope of the invention herein illustrated and
defined in the appended claims. Therefore, while the present
invention has been shown and described herein in what is believed
to be the most practical and preferred embodiments, it is
recognized that departures can be made therefrom within the scope
of the invention, which is not to be limited to the details
disclosed herein but is to be accorded the full scope of the claims
so as to embrace any and all equivalent processes and products.
* * * * *